This invention is directed to the vertical D.C. casting of aluminum, and, in particular, to an improved bottom block design for the vertical D.C. casting of large aluminum rolling ingots. As used herein, aluminum refers to both aluminum and aluminum alloys.
During the initial casting of large aluminum rolling ingots, the bottom surface of the forming ingot in contact with the bottom block begins to curl away from the surface of the bottom block as the metal begins to solidify and contract. This results in the normal curved shape of the butt end of the ingot as shown in FIG. 1. Frequently, as the butt end of the ingot begins to curl away from the top of the bottom block, the forming ingot shell will not be sufficiently strong to support itself and one side of the ingot will start to collapse about a point approximately one-eighth to one-half (usually about one-fourth) of the width of the ingot. This results in a dish-shaped butt surface as shown in FIG. 2. Because of the stress riser left in the solidified ingot having a dish-shaped butt, usually a crack will form at the stress point at the edge of the butt as shown in FIG. 2 which can ultimately extend the entire length of the ingot and thereby require its scrapping.
The formation of dish-shaped butts is a significant problem in casting with steel bottom blocks especially in casting alloys having an intermediate size melting range (e.g., 35.degree.-200.degree.F., particularly 40.degree.-140.degree.F.). With relatively pure alloys, such as 1100 (Aluminum Association alloy designation), the melting range is so narrow that rapid solidification of the butt is assured under normal casting conditions, thereby minimizing the chances of forming a dish-shaped butt. On the other hand, with highly alloyed materials, even though the temperature range between the solidus and liquidus points is broad, the strength of the forming ingot due to the alloying constituents is sufficiently high to preclude the formation of dish-shaped butts.
The dish-shaped butt is a characteristic primarily of steel bottom blocks. In contrast, aluminum bottom blocks provide such a rapid solidification of the butt end of the ingot that the dish-shape is usually prevented from forming. However, because of the rapid solidification promoted by the aluminum bottom block, cold folds and spillouts are created at the corners and end of the ingot butt. These are apparently due to the too rapid extraction of heat from the corners and end of the butt as the ingot curls up from the bottom block and away from the mold too rapidly, thereby allowing the cold folds and spillout to develop.
A typical prior art double curvature steel bottom block is shown in FIG. 3 and a typical aluminum bottom block is shown in FIG. 4 and FIG. 5.
Against this background, the present invention was developed.